Dual effect of cannabinoid CB₁ receptor stimulation on a vanilloid VR1 receptor-mediated response

Abstract.

Cannabinoid CB₁ receptors and vanilloid VR1 receptors are co-localized to some extent in sensory neurons of the spinal cord and dorsal root ganglia. In this study, we over-expressed both receptor types in human embryonic kidney (HEK)-293 cells and investigated the effect of the CB₁ agonist HU-210 on the VR1-mediated increase in intracellular Ca²⁺ ([Ca²⁺]_i), a well-known response of the prototypical VR1 agonist capsaicin. After a 5-min pre-treatment, HU-210 (0.1 μM) significantly enhanced the effect of several concentrations of capsaicin on [Ca²⁺]_i in HEK-293 cells over-expressing both rat CB₁ and human VR1 (CB₁-VR1-HEK cells), but not in cells over-expressing only human VR1 (VR1-HEK cells). This effect was blocked by the CB₁ receptor antagonist SR141716A (0.5 μM), and by phosphoinositide-3-kinase and phospholipase C inhibitors. The endogenous agonist of CB₁ and VR1 receptors, anandamide, was more efficacious in inducing a VR1-mediated stimulation of [Ca²⁺]_i in CB₁-VR1-HEK cells than in VR1-HEK cells, and part of its effect on the former cells was blocked by SR141716A (0.5 μM). Pre-treatment of CB₁-VR1-HEK cells with forskolin, an adenylate cyclase activator, enhanced the capsaicin effect on [Ca²⁺]_i. HU-210, which in the same cells inhibits forskolin-induced enhancement of cAMP levels, blocked the stimulatory effect of forskolin on capsaicin. Our data suggest that in cells co-expressing both CB₁ and VR1 receptors, pre-treatment with CB₁ agonists inhibits or stimulates VR1 gating by capsaicin depending on whether or not cAMP-mediated signalling has been concomitantly activated.